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<meta name="description" content="很多网络问题都跟路由有关，那么首先必须学会看懂路由表，本文将讲述如何读懂路由及如何决策. 在命令行下输入route -n 或 netstat -rn，就可以打印本机的路由表，我的如下： 123456789Destination     Gateway         Genmask         Flags Metric Ref    Use Iface192.168.161.0   192.1">
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<meta property="og:description" content="很多网络问题都跟路由有关，那么首先必须学会看懂路由表，本文将讲述如何读懂路由及如何决策. 在命令行下输入route -n 或 netstat -rn，就可以打印本机的路由表，我的如下： 123456789Destination     Gateway         Genmask         Flags Metric Ref    Use Iface192.168.161.0   192.1">
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          <h1 class="post-title" itemprop="name headline">教你读懂路由表</h1>
        

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        <p>很多网络问题都跟路由有关，那么首先必须学会看懂路由表，本文将讲述如何读懂路由及如何决策.</p>
<p>在命令行下输入<code>route -n</code> 或 <code>netstat -rn</code>，就可以打印本机的路由表，我的如下：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">Destination     Gateway         Genmask         Flags Metric Ref    Use Iface</span><br><span class="line">192.168.161.0   192.168.161.1   255.255.255.0   UG    0      0        0 em1</span><br><span class="line">192.168.161.0   0.0.0.0         255.255.255.0   U     0      0        0 em1</span><br><span class="line">192.168.61.0    0.0.0.0         255.255.255.0   U     0      0        0 em2</span><br><span class="line">169.254.0.0     0.0.0.0         255.255.0.0     U     1002   0        0 em1</span><br><span class="line">169.254.0.0     0.0.0.0         255.255.0.0     U     1003   0        0 em2</span><br><span class="line">192.168.0.0     192.168.61.1    255.255.0.0     UG    0      0        0 em2</span><br><span class="line">0.0.0.0         192.168.61.1    0.0.0.0         UG    0      0        0 em2</span><br><span class="line">0.0.0.0         192.168.161.1   0.0.0.0         UG    0      0        0 em1</span><br></pre></td></tr></table></figure>
<p>PS：我的电脑是双网卡，分别在192.168.61.0和192.168.161.0两个网段</p>
<h3 id="1-字段解释"><a href="#1-字段解释" class="headerlink" title="1.字段解释"></a>1.字段解释</h3><table>
<thead>
<tr>
<th style="text-align:left">字段名</th>
<th>说明</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:left">Destination</td>
<td>目的网段，最长匹配192.168.161.0 &gt; 192.168.0.0 &gt; 0.0.0.0，0可匹配任意数值</td>
</tr>
<tr>
<td style="text-align:left">Gateway</td>
<td>所走网关，0.0.0.0表示无网关，即与本机IP同一网段，不需要经过网关（<strong>同一个局域网内2台主机通信不需要经过网关</strong>）</td>
</tr>
<tr>
<td style="text-align:left">Genmask</td>
<td>掩码</td>
</tr>
<tr>
<td style="text-align:left"><a href="http://www.thegeekstuff.com/2012/05/route-flags/?utm" target="_blank" rel="noopener">Flags</a></td>
<td>标志，U - Up表示有效<br>G - Gateway表示连接路由，若无这个字段表示直连目的地址<br>H - Host表示目标是具体主机，而不是网段</td>
</tr>
<tr>
<td style="text-align:left">Metric</td>
<td>待补充</td>
</tr>
<tr>
<td style="text-align:left">Ref</td>
<td>待补充</td>
</tr>
<tr>
<td style="text-align:left">Use</td>
<td>待补充</td>
</tr>
<tr>
<td style="text-align:left">Iface</td>
<td>网卡</td>
</tr>
</tbody>
</table>
<h3 id="2-路由匹配"><a href="#2-路由匹配" class="headerlink" title="2.路由匹配"></a>2.路由匹配</h3><p>路由表的作用就是指定下一级网关，那么根据路由表怎么确定下一级网关，这里就有一个匹配过程，匹配规则</p>
<p>*（1）优先级匹配（暂不讨论）</p>
<p>*（2）最长匹配</p>
<h3 id="3-实例讲述"><a href="#3-实例讲述" class="headerlink" title="3.实例讲述"></a>3.实例讲述</h3><p>还是针对上面的路由表，为了方便表述，加上条目号字段</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">No  Destination     Gateway         Genmask         Flags Metric Ref    Use Iface</span><br><span class="line">1	192.168.161.0   192.168.161.1   255.255.255.0   UG    0      0        0 em1</span><br><span class="line">2	192.168.161.0   0.0.0.0         255.255.255.0   U     0      0        0 em1</span><br><span class="line">3	192.168.61.0    0.0.0.0         255.255.255.0   U     0      0        0 em2</span><br><span class="line">4	169.254.0.0     0.0.0.0         255.255.0.0     U     1002   0        0 em1</span><br><span class="line">5	169.254.0.0     0.0.0.0         255.255.0.0     U     1003   0        0 em2</span><br><span class="line">6	192.168.0.0     192.168.61.1    255.255.0.0     UG    0      0        0 em2</span><br><span class="line">7	0.0.0.0         192.168.61.1    0.0.0.0         UG    0      0        0 em2</span><br><span class="line">8	0.0.0.0         192.168.161.1   0.0.0.0         UG    0      0        0 em1</span><br></pre></td></tr></table></figure>
<ul>
<li>192.168.61.35 - 匹配条目3，即不需要经过网关</li>
<li>192.168.60.150 - 匹配条目6，需要经过网关192.168.60.1</li>
<li><a href="http://www.baidu.com" target="_blank" rel="noopener">www.baidu.com</a> - 匹配条目7，这里需要说明下为何不匹配8？这是我实践的结果，后加的默认网关会在列表前面，即优先匹配。这个规则应该用于所有Destination一致的情况</li>
<li>192.168.161.113 - 匹配条目1，<strong>不是匹配条目2</strong></li>
</ul>
<h3 id="4-路由编辑"><a href="#4-路由编辑" class="headerlink" title="4. 路由编辑"></a>4. 路由编辑</h3><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># 添加默认网关</span></span><br><span class="line">route add default gw 192.168.1.1</span><br><span class="line"></span><br><span class="line"><span class="comment"># 删除默认网关</span></span><br><span class="line">route del default</span><br><span class="line"></span><br><span class="line"><span class="comment"># 添加本地路由</span></span><br><span class="line">route add -net 192.168.1.0 netmask 255.255.255.0 eth0</span><br><span class="line">route add -net 192.168.1.0/24 eth0</span><br><span class="line"></span><br><span class="line"><span class="comment"># 删除本地路由</span></span><br><span class="line">route del -net 192.168.1.0 netmask 255.255.255.0 eth0</span><br><span class="line">route del -net 192.168.1.0/24 eth0</span><br><span class="line"></span><br><span class="line"><span class="comment"># 清空所有路由</span></span><br><span class="line">ip route flush table main</span><br></pre></td></tr></table></figure>
<h3 id="5-总结"><a href="#5-总结" class="headerlink" title="5. 总结"></a>5. 总结</h3><ul>
<li>结论1： 一台局域网下的主机路由表至少需要包含一条本地路由，否则无法与路由网关通信，当添加默认网关，会提示”SIOCADDRT: Network is unreachable”。</li>
</ul>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">No  Destination     Gateway         Genmask         Flags Metric Ref    Use Iface</span><br><span class="line">2   192.168.161.0   0.0.0.0         255.255.255.0   U     0      0        0 em1</span><br></pre></td></tr></table></figure>
<p>这样才能ping通网关（192.168.161.1）及外部ping通本机</p>
<ul>
<li>结论2： 一台局域网下的可访问Internet主机路由表至少需要包含两条路由，一条是本地路由，一条是默认网关</li>
</ul>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">No  Destination     Gateway         Genmask         Flags Metric Ref    Use Iface</span><br><span class="line">2   192.168.161.0   0.0.0.0         255.255.255.0   U     0      0        0 em1</span><br><span class="line">7   0.0.0.0         192.168.61.1    0.0.0.0         UG    0      0        0 em2</span><br></pre></td></tr></table></figure>

      
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